Method for masking and removing stains from rugged solid surfaces

ABSTRACT

Methods, compositions and kits for masking and subsequent removal of oil, grease, rust and other stains from a variety of rough solid surfaces, including, but not limited to stone, concrete, asphalt, stucco brick, and ceramic. The methods include coating the stains with an opaque or translucent thin film of a composition that makes the stain practically indistinguishable against the background, exposing the coated stain to the elements, such as, sunlight, air, moisture, resulting in spontaneous transformation, degradation and subsequent removal of the stains from the solid surfaces. The compositions include a photocatalyst by itself or the photocatalyst combined with at least one of, a sensitizer, a dopant, a mediator, a co-reagent, a pigment and a binder. The role of a photocatalyst is to produce highly reactive species or radicals and initiate the degradation of a stain upon exposure to elements, such as, sunlight, air and ambient humidity. Artificial light sources can be used instead of sunlight.

This invention relates to chemical methods of stain removal from solidsurfaces, and in particular to a photocatalytic methods, compositionsand kits for masking and removing stains, such as oil, grease, paint,mold, rust and the like from rugged solid surfaces, such as stone,asphalt, stucco, concrete, brick, ceramics, and the like.

BACKGROUND AND PRIOR ART

Stains are unsightly discolored or soiled spots or smudges that occurpractically anywhere. Common sources of stains are oil, grease, paint,rust, mold, plants, food and beverages, bird droppings, tire marks, andthe like. In most cases, a stain is a result of a chemical reaction orstrong interaction, such as, adsorption or chemisorption, between astaining agent and the surface of a solid material.

Lubricating oil (lube-oil) stains on concrete and asphalt driveways areamong the most common stains and, unfortunately, most resistant stains.This is due to their high molecular weight and viscosity, low solubilityin common solvents and pronounced hydrophobic properties. Chemically,lube-oils present a very complex mixture of long-chain hydrocarbons(>C₂₀) and a number of additives including oxidation inhibitors (toprolong the life of the lube-oil), anti corrosion agents (to protectmetal from sulphides and other corrosive elements), guards againstscoring and galling, anti wear agents (to prevent abrasive and metal tometal contact) and others.

Used lube-oil also contains heavy metals, such as, nickel (Ni), copper(Cu), chromium (Cr), vanadium (V), oxygenates, sulphur compounds andother impurities. Due to an intrinsic chemical inertness of lube-oils,the requisite property of the lube-oil, and the presence of an oxidationinhibitor a lube-oil stain may remain intact for an extended period oftime, months and even years. All these factors add to the difficulty oftreating and removing lube-oil stains, especially, from rough surfaces.

A rust stain is another common and unforgiving adversary for concreteand other rugged surfaces. Rust stains mainly result from two sources. Afirst source of rust is water running over metal such as iron and steeland depositing metal oxide particles on the paving or other surfaces. Asecond source is oxidation of iron-rich compounds in the aggregate ofthe paving or other surfaces. In principle, rust can be removed viareactions with phosphoric, muriatic, oxalic acids and other acids. Mostcommercial rust removers contain acids. However, any chemical agent,especially, acid-based, that treats the rust stain would potentiallyaffect the material of the surface sometimes leading to itsdiscoloration or even damage. The stains are particularly difficult toremove from rugged, rough porous surfaces such as those typicallyoccurring in concrete, brick, limestone and other materials. Cement is aversatile and widely used paving and building material, but it is porousand has a very high surface area as reported by V. Ramachandran, et al.in “Removal of Stains from Concrete Surfaces,” Canadian Building Digest.Report CBD-153, (1973). Stains tend to soak into cement on contact andin some cases may react with the hydrated cement. Some stains,particularly old ones, so strongly adhere to the porous surface ofcement that they will resist to any efforts to remove them. As a result,removal of old oil stains from rugged surfaces like those occurring oncement, brick and other porous materials becomes extremely difficult andrequires a special treatment.

Known procedures for removing stains from rugged solid surfaces, suchas, concrete, include mechanical and chemical methods or a combinationof both methods. Mechanical methods include sand blasting, grinding,steam cleaning, brushing, scouring and use of blow torches. In mostcases, these procedures are cumbersome, labor intensive and ratherexpensive, and do not always give a desirable cleaning effect. Dependingon the age and severity of the stain these methods may require multipletreatments to get results, which could potentially compromise theintegrity of the surface subjected to cleaning.

Existing chemical methods of stain removal from solid surfaces involvethe use of special chemicals, solvents, detergents, enzymes and otherbiological agents. Solvents and detergents dissolve stains, whereaschemical and biological agents react with them leading to theirdegradation or formation of a compound that will not show as a stain.

The prior art describes a variety of chemical methods for removingorganic stains from solid surfaces. For example, Matsui et al. inJapanese Patent JP 10176427 teaches a method for removing oil stain fromstones. The method includes coating the oil stain with a pastecontaining a lipophilic organic solvent and solid powders, covering thesurface of the stone by a film, which does not permeate vapors of thesolvent, adhering the covering with tapes to prevent the vapor fromscattering.

Japanese Patent JP 08188893 to Asakawa describes aqueous detergents forremoving oil stains from metal objects. The method involves the use ofaqueous detergents containing fat-decomposing enzymes and nonionicsurfactants at a ratio of (1-100):(1-1000) and have pH 6.5-10.

European Patent EP 441481 to Kerze discloses a method for cleaninghydrocarbon-stained hard surfaces. The method includes surrounding thestain on the surface with means for containing a liquid in contact withthe stain, placing in the containment an organic solvent that ismiscible with petroleum hydrocarbons, maintaining the solvent in contactwith stain for 1-24 hours, removing the solvent, placing on the surfacean adsorbent that does not dissolve the solvent, maintaining theadsorbent in contact with the surface for an additional 1-24 hours,removing the adsorbent material and allowing the treated surface to dry.The author claims that the method is effective in removing motor oilstains from concrete surfaces.

An absorbent composition for removing oils and greases from driveways isdescribed by Hatton in European Patent EP 260135. The absorbentcomposition comprises an organic material derived from plant gums andplant mucilages and it is applied as a powder, paste or sheet to theliquid spill or the layer to be absorbed. Thus, ground Plantago psylliumhusk powder was applied to a patch of oil on a concrete surfaceresulting in removal of the oil after several hours.

Canadian Patent CA 978839 to Parent describes the liquid compositionsfor removing rust stains from concrete, bricks and ceramics. Thecomposition consists of concentrated phosphoric acid (H₃PO₄) (98 wt. %)diluted after mixing with ≦0.5% of 1,3-dibutylthiourea and/or1,3-diethylthiourea and ≦0.1% of a nonionic surfactant.

Uemura in Japanese Patent JP 52069827 teaches a method for rust removalusing a paste-like agent. The agent comprises silicon dioxide (SiO₂)(50-95%), water and/or alcohol, surfactant and a H₃PO₄ compound. Therust-removing agent is applied to a metal surface, solidified after 5-10hours, and easily removed the rust by brushing.

An overview of different methods for removing oil and rust stains frommortar and concrete specimens was reported by Derrington et al.“Removing stains from mortar and concrete.” U.S. Govt. Res. Develop. Rep1969, 69/5, 88). It was reported that the most effective method for theremoval of rust stains is sandblasting and an application of an oxalicacid solution or sodium citrate-sodium dithionite paste. An applicationof benzene-calcium carbonate (C₆H₆—CaCO₃) paste was reportedly the mosteffective removal method for oil stains among the techniques that weretried. The report also stated that neither of the known methods couldcompletely remove oil and asphalt stains from concrete surfaces.

Thus, the existing chemical approaches to removal of resistant stainsfrom rugged solid surfaces offer rather labor-intensive and costlytechniques that involve the use of expensive organic solvents,biological agents (enzymes), special detergents, surfactants, orcorrosive agents, such as acids. Moreover, some solvents, such as,benzene, and acid-based agents may have an undesirable environmentalfootprint. Furthermore, after the stain treatment by the agents,including solvents, detergents, acid-based formulations, and the like,they have to be washed away by copious amounts of water to prevent asecondary staining.

It should also be emphasized that most of the fore-mentioned chemicalmethods, especially, those involving the use of poultices or acid-basedformulations, are designed for the treatment of horizontal surfaces andmay not be applicable for vertical surfaces. In summary, these methodsare cumbersome and costly and in many cases do not give good results,especially in treatment of old lubricant-oil and asphalt stains, makingthem temperamental or even impractical for some applications.

Turning now to photocatalytic methods for the removal of organiccontaminants and oil spills from a variety of solid and liquid (e.g.,seawater) surfaces, the prior art provides the following examples.

U.S. Patent Publ. No. 2004/0149307 to Hartig describes a self-cleaningreversible window assembly with photocatalytic (e.g., TiO₂) layersdeposited on opposite sides of a transparent substrate (glass) so thatwhen exposed to sunlight, the photocatalytic layers chemically degradeorganic contaminants deposited on the transparent surface. Similarly,U.S. Pat. No. 6,680,135 to Boire et al. relates to transparent surfaces(e.g., glass), which are furnished with photocatalytic coatings thatimpart “dirt repellent” properties to the surface.

The transparent TiO₂ photocatalytic coatings according to Hartig's andBoire's inventions are produced from titanium-based precursor compoundsand would require special equipment and sophisticated applicationtechniques such as magnetron sputtering, pyrolytic coating, chemicalvapor deposition, cathodic sputtering, sol-gel techniques, etc. Thesemethods, however, may not be suitable for the application ofphotocatalytic coatings on out-door surfaces, such as concretedriveways, asphalt pavement, etc.

U.S. Patent Publ. No. 2004/0255973 to Chen discloses a method forcleaning the surface of a semiconductor wafer (after chemical/mechanicalpolishing) comprising the steps of applying a photocatalyst (e.g., TiO₂)containing suspension to the surface and exposing it to UV light(wavelengths less than 380 nm).

U.S. Pat. No. 6,645,307 to Fox et al. discloses cleaning compositionscomprising a photocatalytic material (colloidal suspensions of TiO₂), asensitizer (e.g., cationic dye/borate anion complex) and a number ofother ingredients. The composition combats soils and/or undesiredmalodours and microorganisms on fabrics and hard surfaces. The TiO₂colloidal suspensions are produced from precursor materials, e.g., byhydrolysis of tetrachloride or titanium isopropoxide. Similarly, U.S.Patent Publication No. 2004/0023824 to Zeuchner et al. applies insolublesolids such as silica, zinc oxide or TiO₂ in solution to hard surfacessuch as stone, ceramics, wood and the like to increase hydrophilicproperties of the surface so that the removal of soil and soil-releaseare improved.

U.S. Pat. No. 6,013,372 to Hayakawa, et al. describes a surface coatedwith an abrasion-resistant photocatalytic coating comprised of asemiconductor photocatalyst, such as TiO₂. The coating, when exposed tosunlight, makes the surface hydrophilic and self-cleaning when subjectedto rainfall.

U.S. Pat. No. 6,659,520 to Jacobs provides a coating composition forpreventing algal growth on building materials. The coating compositionis an aqueous slurry containing a silicate binder (e.g., sodiumsilicate) and a plurality of photocatalytic (e.g., TiO₂) particles(particle size 1-1000 nm). The composition is heated, up to 650° C., toproduce a ceramic-type coating on granular substrates such as rock,clay, ceramic, concrete, and the like.

U.S. Pat. No. 6,409,821 to Cassar et al. describes a hydraulic binderand cement compositions containing photocatalyst (e.g., TiO₂) particleshaving the improved property of maintaining brilliance and color forarchitectural concretes. The amount of photocatalyst particles in thecomposition varied in the range of 0.01-10% by weight with respect tothe hydraulic binder.

U.S. Pat. No. 6,337,129 to Watanabe et al. discloses a surface layercomposition comprising a photocatalyst (e.g., TiO₂), a hydrophobicportion (a water repellent fluororesin) and a hydrophilic portion (asilicone resin or silica) in a solvent (water, ethanol, or propanol).The mixture is spray-coated on the substrate surface and the coating isheat treated at 200-380° C. to form a durable surface layer.

U.S. Pat. No. 5,643,436 to Ogawa et al. describes a metal oxide (e.g.,TiO₂) layer exhibiting a photocatalytic and anti-soiling function on thesurface of inorganic architectural materials (e.g., glass, tile,concrete, stone, etc.). Transition metals (Pt, Pd, Ni, Ru, etc.) ortheir oxides can be added to promote the photocatalytic reaction. Heattreatment (100-800° C.) is necessary to bind the photocatalyst film tothe architectural material.

U.S. Pat. No. 5,547,823 to Murasawa et al. discloses a photocatalystcomposite comprising a substrate with the photocatalyst (TiO₂) particlesand the second component (V, Fe, Co, Ni, et.) adhered to a solid surface(glass, ceramics, wood, metals, etc.) using less degradative adhesive (afluorinated polymer, a silicon-based polymer, cement, etc.). TiO₂particles are produced from precursors such as titanyl sulfate, titaniumtetrachloride, titanium alkoxides and the like by a number oftechniques: thermal hydrolysis, neutralization, vapor-phase oxidation.The fixation of the composition to the substrate is accomplished byUV-irradiation, thermal treatment (at temperatures up to 400° C.), usinga cross linking agent, etc.

Schutt in WO 2003022462 describes titanium alkoxide-polysiloxane-basedcoatings with photocatalytic self-cleaning properties. A substrate iscoated with a polysiloxane-based coating having titanium oxy-groups froma titanium alkoxide bonded to the polysiloxane backbone. The coatinggives the substrate self-cleaning properties upon exposure to UVradiation causing organic stains to spontaneously disappear. Uponexposure to UV radiation, photocatalytically induced oxidation reactioncauses the spontaneous disappearance of food, oil and other organicstains on buildings and paving materials, such as concrete, limestone orsimilar surfaces.

Japanese Patent JP 2000015110 to Nakamura et al. describes the methodfor the removal of stains on kitchenware. The method utilizes thephotocatalyst particles comprising TiO₂ grains with an average diameterof equal or less than 1 μm which are partially coated (1-20%) by a metalor its alloy selected from Ru, Pt, Au, Cr.

U.S. Pat. No. 5,604,339 to Raissi et al discloses a method forphotocatalytic destruction of harmful volatile compounds at emittingsurfaces. In a preferred embodiment, a piece of formaldehyde laden woodsubstrate, for example, paneling or furniture, is treated with TiO₂suspension to form a thin and translucent veneer, which acts as amembrane preventing outward transport of formaldehyde and other harmfulcompounds produced by weatherization and natural degradation of thesubstrate.

In addition to the above, another series of patents deal with thephotocatalytic degradation of oil spills on seawater. For example, U.S.Pat. No. 4,997,576 to Heller et al. discloses the method forphotocatalytic treatment of an oil film (or spill) floating on seawater,which includes dispersing TiO₂-coated water-floatable beads (an averagediameter of ≦2 mm) on the oil film and allowing the beads to be exposedto solar illumination and air so that the beads accelerate the oxidationof organic compounds in the oil.

Japanese Patent JP 11300349 to Tokita describes a method for cleaningwater surface contaminated by oil spill. The method is carried out byfloating on the contaminated water surface a water-soluble substratecoated with thin-film semiconductor photocatalyst and porous material(e.g., zeolite), to be contacted with oil pollution for decompositionunder light (solar or lamp) radiation.

Japanese Patent JP 10310779 to Takutani describes photocatalyst devicesfor treatment and recovery of oil spill on seawater. The devices aremade of honeycomb-type photocatalysts having multiple cell holes andTiO₂ coatings, and the devices are equipped with floats. Oil spills arephotocatalytically oxidized and decomposed under solar light radiation.

In summary, the prior art methods offer a variety of approaches, themajority utilize TiO₂ as a photocatalyst, to degrade and remove organiccontaminants and stains from different surfaces. Potential problemsassociated with these approaches are as follows. First, most of thetechniques utilize rather costly starting materials, such as,metal-organic precursors, polymeric adhesives, micro-beads, and thelike. Second, the majority of methods used require special equipment andelaborate preparation techniques to produce the photocatalytic coatings,including, but not limited to, metal-organic synthesis, chemical vapordeposition, magnetron sputtering. Third, in many cases, the binding ofthe photocatalyst to the treated surfaces requires thermal treatment(100-600° C.), which is not technically feasible for out-doorapplications in locations, such as, driveways, pavements, exteriorwalls.

Moreover, the prior art techniques are not conducive to the degradationof organic contaminants occurring on rugged porous surfaces, where anintimate contact between a photocatalytic agent and the stain surface,including the portion hidden in the micro-pores and micro-crevices, isrequired. Furthermore, there is no indication that the prior art methodscould provide an immediate (i.e., right after the initial treatment)masking effect on the stains making them less visible against thesurroundings. Lastly, the above photocatalytic techniques are designedfor the treatment of different organic stains and are unlikely to treatand remove inorganic stains, such as those caused by rust.

It is also known that conventional paints contain titanium dioxide andother pigments. However, conventional paints are not useful for maskingand removing stains for several reasons. First, paints can provide onlymasking effect, but they do not remove the stains. Second, paintscontain a vehicle (such as oil- or lacquer- or latex-based vehicles), inwhich the pigment is suspended. The vehicle dries and hardens to form anadhesive film of paint on the surface. This film is practicallypermanent and over extended period of time (months and years) itsmasking effect might diminish as the surroundings change color due toaging and exposure to traffic. Thus, paints are not useful for maskingand removing stains.

Thus, there is a need for an efficient, simple, user-friendly, care-freeand environmentally benign method for the removal of a variety of toughstains from rugged porous surfaces (both horizontal and vertical) suchas concrete, stone, brick, stucco, and the like. It is also highlydesirable that the method provides an immediate masking effect on thestain via blending the stain with the surrounding area and making itpractically invisible.

SUMMARY OF THE INVENTION

A primary objective of the present invention is to provide methods,compositions, and kits for removal of oil, grease, paint, mold, rust andother tough stains from rough porous solid surfaces such as but notlimited to concrete, stone, asphalt, stucco, brick, ceramics, and thelike.

A secondary objective of the present invention is to provide methods andkits that can include a composition that can be applied to a stain inthe form of a thin opaque or translucent film providing a masking effecton the stain.

A third objective of the present invention is to provide methods,compositions and kits for making a stain on a rough porous solid surfacebecome practically invisible against the background.

A fourth objective of the present invention is to provide a compositionthat when applied to a stain is capable of facilitating itstransformation and degradation upon exposure to the elements, includingsun, air, rain and ambient humidity.

A fifth objective of the present invention is to provide a kit that canbe used to mask and/or remove stains from rugged, solid surfaces.

A preferred method is disclosed for masking and subsequent removal ofoil, grease, rust and other stains from a variety of rough solidsurfaces, including, but not limited to, stone, concrete, brick, andwood. The method includes coating the stains with an opaque ortranslucent thin film of a composition that makes the stain practicallyindistinguishable against the background, exposing the coated stain tothe elements, such as, sunlight, air, moisture, resulting in spontaneoustransformation, degradation and subsequent removal of the stains fromthe solid surfaces.

The composition can include a photocatalyst by itself, or thephotocatalyst with one or more components selected from the following: asensitizer, a dopant, a mediator, a co-reagent, a pigment and a binder.The role of a photocatalyst is to produce highly reactive species orradicals and initiate the degradation of a stain upon exposure to theelements, such as, sunlight, air and ambient humidity. Artificial lightsources can be used instead of sunlight; the light wavelength of theartificial light source is preferably in the range from approximately250 nanometers (nm) to approximately 800 nm, more preferably,approximately 365 nm to approximately 580 nm.

The role of a sensitizer is to extend the light absorption ofUV-sensitive photocatalysts to visible area of the solar spectrum, and,thus, enhance the photocatalytic action of the materials.

The role of a dopant is to facilitate oxidative degradation of the stainconstituents via redox transformations in the presence of air and water.

The role of a mediator is to facilitate the contact between theconstituents of the stain and the photocatalyst.

The role of a co-reagent is to transform the constituents of the staininto a form that could be more efficiently degraded or will not show asa stain.

The role of a pigment is to impart to the composition a color that willmask the stain against the surroundings until the stain completelydegrades and disappears from sight.

The opacity of the thin film is controlled by the nature of thephotocatalyst and the pigment used, size of particles, thickness of thefilm. For example, titanium dioxide (TiO₂) nanopowder (Degussa P25) withan average particle size of approximately 30 nm in diameter producestranslucent films, whereas, TiO₂ with an average particle size ofapproximately 1 micron (μm) produces opaque films. Adding pigments, suchas Fe₂O₃, increases the opacity of the film. Likewise, increasing thethickness of the film increases the opacity.

The components in the thin film can be varied, other than thephotocatalyst, the other ingredients are used as desired and in variedamounts, by a person of skill in the art to vary the functionality ofthe composition. In some cases, the composition could consist of onlyone ingredient, a photocatalyst suspended in a solvent or in the matrixof a binder.

In many cases, the same component of the composition can perform severalfunctions. For example, the same component can act as a photocatalystand as a pigment. The disclosed method includes applying a durable thinfilm of the composition that is stable under exposure to elements, suchas, sun, wind, rain and mechanical action (e.g., walking or driving overthe film) for an extended period of time of weeks and months, duringwhich the stains usually disappear from sight.

Follow-up treatment may be required if the coating is weathered out byrain or heavy traffic before the stain is completely removed. When astain is not completely removed, a new coating or thin film of thecomposition needs to be applied over the stain surface. There is also apossibility that the coating may still be visible after the stain isremoved; if so, the residual coating could be removed by hosing orbrushing with soap and water.

The method of the present invention is particularly advantageous for themasking and/or removal of old tough stains that are most difficult orsometimes, impossible to remove by conventional techniques and existingcommercial products. The difficult stains include, but are not limitedto, old lube-oil stains, tire marks on concrete, asphalt driveways andpavements.

Further objects and advantages of this invention will be apparent fromthe following detailed description of the presently preferredembodiments, which are illustrated schematically in the accompanyingdrawings.

BRIEF DESCRIPTION OF FIGURES

FIG. 1. is a schematic representation of the composition film applied tothe surface of an oil stain occurring on a substrate.

FIG. 2A is an illustration of an area of concrete with an original oilstain on the surface.

FIG. 2B is an illustration of the oil stain in FIG. 2A immediately aftercoating with a thin film of the composition of the present invention.

FIG. 2C is an illustration of an area of concrete after the coated stainhas been exposed to the elements for one month.

FIG. 3 is an illustration of a stain masking and removing kit of thepresent invention.

FIG. 4A is a photograph of a lube-oil stain on an asphalt drivewayimmediately after applying the TiO₂ film as a liquid suspension.

FIG. 4B is a photograph of the lube-oil stain on the asphalt driveway inFIG. 2A after treatment and 5 months of exposure to weather and traffic.

FIG. 5A is a photograph of a lube-oil stain on a concrete drivewaybefore application of the composition film as a liquid suspension.

FIG. 5B is a photograph of the lube-oil stain on the concrete drivewayafter treatment and exposure to the elements for one month.

FIG. 6A is a photograph of a lube-oil stain on an asphalt drivewaybefore applying thin film of the present invention as a solidcomposition.

FIG. 6B is a photograph of the lube-oil stain on an asphalt drivewayimmediately after applying the solid form of the composition as a thinfilm.

FIG. 7 is a photograph of a tire mark on an asphalt driveway with theportion of the mark treated with the solid form of the composition ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the disclosed embodiments of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangements shown sincethe invention is capable of other embodiments. Also, the terminologyused herein is for the purpose of description and not of limitation.

The present invention provides a method for masking and subsequentremoval of a variety of stains, such as, lubricating oil, grease, rust,mold and the like, from rugged porous surfaces, including, but notlimited to, stone, concrete, brick and stucco with improved efficiency,ease and convenience compared to conventional techniques. The methodincludes coating the stains with an opaque or translucent thin film of acomposition that makes the stain difficult to distinguish from thebackground, and exposing the film-coated stain to an outdoor environmentwith sunlight, air, ambient humidity, resulting in spontaneoustransformation, degradation and subsequent disappearance of the stainsfrom a variety of solid surfaces.

To be suitable for stain masking and removal, the composition of thepresent invention has to meet the following requirements. First, thecomposition has the ability to induce photo-oxidation reactions uponexposure to ultraviolet (UV) and/or visible light. Second, thecomposition has high photo- and chemical stability. Third, thecomposition has the ability to produce a durable thin film with highopacity and good adhesion to different surfaces, including hydrophobicsurfaces.

There are two preferred embodiments of the present invention, a liquidphase composition and a solid phase composition. According to a firstembodiment, the composition can include one or more components selectedfrom a photocatalyst, a sensitizer, a dopant, a mediator, a co-reagentand a pigment suspended in a solvent.

According to a second embodiment, the composition can include thephotocatalyst combined with one or more components selected from asensitizer, a dopant, a mediator, a co-reagent, a pigment and a binder.

The photocatalyst can be selected from the group of compounds containingzinc (Zn), titanium (Ti), molybdenum (Mo) and tungsten (W). Thephotocatalyst can also be selected from one of titanium dioxide (TiO₂),zinc oxide (ZnO), and a mixture of TiO₂ and ZnO.

The method for applying the two embodiments of the present inventionrequires the application of a thin film of the composition onto thestain surface. In a first embodiment, a thin film is formed by applyinga liquid suspension of the composition in a solvent to a stain andsubsequently, vaporizing the solvent. Vaporizing occurs when exposed toambient or outdoor temperatures and pressures.

The application of the liquid suspension to a stain can be accomplishedby spray-coating, dripping, or sprinkling the composition onto the stainsurface, or by soaking a sponge or a cloth, or other liquid-absorbingmaterial with the suspension and rubbing it on the stain surface.

Any technique that can produce a thin film over the stain surface isacceptable, for example, coating with a brush, sponge, cloth,sprinkling, sputtering, spraying from a pressurized can and the like. Itis preferable that the liquid suspension be applied in such a mannerthat it exactly covers the footprint of the stain. This could beachieved by starting the application in the middle of the stain andslowly moving outward.

In a second embodiment, a solid composition in the form of a loosepowder, pellets, or a solid bar is applied to the stain surface bysputtering or, in the case of a bar, by shading followed by rubbing thecomposition onto the stain surface with a sponge, cloth or otherliquid-absorbing material wetted with a solvent. Rubbing a solid bar issimilar to using chalk on an asphalt or concrete surface.

A solvent is used in both embodiments of the present invention and caninclude water, organic solvents or water-organic solutions. Non-limitingexamples of an organic solvent include ethanol, isopropanol, acetone,octane, and mixtures thereof. The solvent does not have a chemicalfunction in the removal or masking of a stain. The role of a solvent isto help create a uniform thin film or coating on the surface of thestain.

The role of a photocatalyst is to initiate degradation or decompositionreactions of organic compounds composing the stain upon exposure to theelements, such as sunlight or an artificial light source, air andmoisture or ambient humidity. The degradation of staining agents occursvia interaction of the organic molecules in the stain with the radicalsand/or other active species produced by the photocatalyst upon exposureto the elements. The photocatalyst is selected from a number ofsemiconductors (SC) or metal complexes capable of absorbing ultraviolet(UV) and/or visible photons and producing reactive species participatingin photo-degradation reactions. Non-limiting illustrative examples ofphotocatalysts include zinc oxide (ZnO), titanium dioxide (TiO₂),strontium titanate (SrTiO₃), tungsten oxide (WO₃), silicotungstic acid(H₄SiW₁₂O₄₀) and other materials, preferably, ZnO and TiO₂.

Current scientific theory holds that upon exposure to photons, air andmoisture, SC-based photocatalysts (e.g., TiO₂) generate a variety ofreactive oxidizing species shown in chemical reaction (1):TiO₂+photons+O₂+H₂O→(TiO₂)+OH., HO₂., H₂O₂  (1)These oxidizing species are capable of attacking organic moleculescausing destructive oxidation and, ultimately, decomposition intosmaller molecules such as carbon dioxide (CO₂), carbon monoxide (CO),methane (CH₄), water (H₂O), predominantly, CO₂ and H₂O, as shown inreaction (2):OH.+C_(x)H_(y)O_(z)+O₂→→→CO₂+H₂O  (2)where, C_(x)H_(y)O_(z) denotes an organic molecule and →→→ the sequenceof reactions.

In the case of hydrocarbon-based staining agents, such as, lube-oil, thestaining agents are initially photooxidized into oxygenated compounds,such as, alcohols, ketons, or organic acids, as shown in reaction (3):OH.+C_(n)H_(m)+O₂→C_(n-x)H_(m-y)O_(z)  (3)where, C_(n)H_(m) and C_(n-x)H_(m-y)O_(z) are the original andoxygenated long-chain hydrocarbons, respectively.

The oxygenated reaction products have higher solubility in water thanthe original hydrocarbons, and consequently, can be more easily washedout from the surface by rain or water-hosing, thus facilitating thedisappearance of the stain from the surface.

Several metal-complex photocatalysts, such as, polyoxometalates (POM),are also capable of oxidative degradation of organic compounds in thepresence of molecular oxygen when exposed to solar photons. POM aretransition-metal oxygen-anion clusters with general formula of[X_(x)M_(m)O_(y)]^(q−), where M is usually molybdenum (Mo) or tungsten(W), less frequently, vanadium (V), niobium (Nb), tantalum (Ta) and X isa heteroatom.

Of particular interest are POM of tungsten (e.g., [XW₁₂O₄₀]^(q−), whereX is P, Si, Fe, and q=3 through 7) that photocatalyze the oxidation of awide range of organic compounds, such as, alcohols, esters, organicacids, and the like. Papaconstantinou et al, Homogeneous andHeterogeneous Photocatalysis, D. Reidel Publ. Co., Dordrecht, pp. 415-31have reported the oxidation reactions of organic compounds. Muradov etal. reported on the solar photocatalytic decomposition of nitrate estersin the presence of H₄[SiW₁₂O₄₀]—TiO₂ binary system in Amer. Chem. Soc.,I&EC Sixth Annual Symposium on Emerging Technologies in Hazardous WasteManagement, Atlanta, Ga., (1994).

The mechanism of POM-catalyzed photodegradation of organic compounds isdifferent from that of semiconductor-catalyzed reaction and involves thephotoinduced charge transfer in the photoactive W^(V1)=O group resultingin the formation of a reactive electron-deficient radical-like specie(W^(V)=O.) capable of attacking organic compounds, as shown in reactions(4) and (5) below:W^(V1)=O+photons→W^(V)=O.  (4)W^(V)=O.+C_(x)H_(y)O_(z)+O₂→→→W^(V1)═O+CO₂+H₂O  (5)where, W^(V1)=O is a fragment of POM molecule.

Similar to TiO₂ and ZnO, POM are sensitive to UV photons withwavelengths less than 400 nm and can accomplish solar-catalyzeddegradation of organic compounds. Thus, in principle, both types ofphotocatalysts, semiconductors and metal-complexes are applicable to thetreatment of outdoor as well as indoor stains. In the case of indoorstains, light sources such as conventional black light and cool whitefluorescent lamps can be used.

The role of a sensitizer is to extend the absorption band ofUV-sensitive photocatalysts (e.g., TiO₂, ZnO) into visible area of thesolar spectrum, and, thus, enhance the photocatalytic action of thematerials. The band gaps of TiO₂ (rutile form), TiO₂ (anatase form) andZnO are 3.0, 3.2 and 3.2 eV, respectively, which makes them sensitive tonear-UV radiation, for example, wavelengths equal or less than 385 nmpresent in solar spectrum (about 5% of the total spectrum).

It is possible, however, to sensitize these semiconductors to visiblephotons with wavelengths more than 400 nm and, thus, increase the rateof solar-assisted decomposition of organics. For example, the lightabsorption by TiO₂ nanoparticles doped with carbon can be extended wellinto the visible area as discussed by Kahn et al., Science, v. 297, p.2243, 2002. A similar effect was reported by Li and Haneda inChemisphere, v. 54, p. 1099, (2004) who found that the sensitivity ofZnO photocatalyst toward visible light could be enhanced by doping itwith nitrogen.

Due to the strong oxidizing action of the UV-excited photocatalysts, thesensitizers have to be resistant to oxidation and photodegradation.Non-limiting examples of a sensitizer include metal-organic complexesand dyes such as tris(2,2′-bipyridyl)ruthenium (II) chloride,cis-RuL₂(SCN)₂ (where, L is 2,2′-bipyridyl-4-4-dicarboxylate), porphirinand phthalocyanin complexes of metals, including, but not limited tozinc (Zn), iron (Fe), copper (Cu). The sensitizers have to beimmobilized on the surface of the photocatalyst prior to mixing it withother components of the composition.

Light sources other than sunlight could also be used for the excitationof the photocatalysts. These sources include low, medium and highpressure mercury, xenon arc, metal halide, fluorescent black light withemission maximum at about 365 nm and cool white fluorescent light withpeak output at about 580 nm lamps.

The role of a dopant is to facilitate the oxidation and degradationreactions involving stain components via redox transformations in thepresence of oxidants, such as, oxygen, hydrogen peroxide, organicperoxides, and the like. Transition metals and their compounds,including oxides and salts are among most efficient dopants.Non-limiting examples of a dopant include Pt, Pd, Ag, RuO₂, Fe₂O₃,Fe₃O₄, FeCl₃, CuO, CoO, SnO₂, Zn(OAc)₂. Dopants facilitate thephotodegradation reactions by catalyzing the electron-transfer reactionsbetween the photocatalyst and organic molecules, and participate innon-photolytic redox reactions leading to formation of reactive species.The last factor could play a major role in additional formation of OHradicals from hydrogen peroxide (H₂O₂) in the presence of iron-baseddopants via Fenton reaction (6) below:H₂O₂+Fe²⁺OH.+OH⁻+Fe³⁺  (6)The importance of this reaction is that it can occur in the areas, suchas pores and crevices, not reachable by photons.

The role of a mediator is to facilitate the contact between theconstituents of a stain, a photocatalyst, air and water, and, thus,facilitate the degradation reaction. Non-limiting illustrative examplesof a mediator include molecular sieves (zeolites), silica gel, activatedcarbon (AC), carbon black (CB). The accelerating effect of zeolites,silica gel and AC could be attributed to their ability to adsorb andretain relatively large amount of water, thus promoting reaction (1)supra. The positive effect of AC and CB can be explained by theirhydrophobic properties, which in some cases (e.g., lube-oil) facilitatesthe interaction of the stain with the reactive species.

The role of a co-reagent is to transform the constituents of the staininto a form that can more efficiently degraded or not show as a stain.Non-limiting examples of a co-reagent include oxalic acid, citric acid,sodium citrate, potassium hydrogen tartrate, sodium phosphate, sodiumhydrosulfite. The function of a co-reagent is best illustrated in thecase of a rust stain removal. In this case, a co-reagent (e.g., oxalicacid) reacts with the hydrated ferric oxide, FeO(OH), the main componentof a rust stain, converting the rust stain into water-soluble ferricoxalate, Fe₂(C₂O₄)₃. Under exposure to solar radiation, ferric oxalateis photo-degraded into iron oxide in the presence of a photocatalyst,such as, TiO₂, as shown in reactions (7):½Fe₂(C₂O₄)₃+TiO₂+O₂+photons→FeO/TiO₂+3CO₂  (7)

As a result, the staining agent (i.e., hydrated iron oxide) istransformed and transferred from the treated surface onto the TiO₂surface. It should be noted that oxalic acid alone can also remove arust stain (see the discussion of a prior art), but in this case theresulting iron oxalates have to be immediately washed away from thesurface by copious amount of water to prevent a secondary staining andalso to remove the unreacted oxalic acid. In contrast, the disclosedmethod does not require immediate washing of the treated area. After thedeposition of iron oxides on TiO₂ surface, the deposits will become lessvisible compared to the original stain, and eventually they will weatherout leaving behind the rust-free surface, and the unreacted oxalic acidwill be photocatalytically decomposed into carbon dioxide (CO₂) andwater (H₂O).

The role of a pigment is to impart to the composition a color that willmask the stain (i.e., by blending it in the surrounding area) until itcompletely degrades and disappears from a sight. Non-limiting examplesof a pigment are: TiO₂, ZnO, Fe₂O₃, Fe₃O₄, NiFe₂O₄, activated carbon,carbon black, etc. Both TiO₂ and ZnO have very high refractive index(which determines the opacity), that makes them two of the most widelyused white pigments in paints, ceramics, cosmetics, paper, and the like.

Due to their high opacity, relatively thin films of these compoundswould provide an adequate masking effect for most of the dark-coloredstains, such as, lube-oil, tire marks, mold, and the like. In somecases, the TiO₂- or ZnO-based coatings may be too bright (especially, ona gray concrete surface), which will necessitate the addition of otherpigments to provide a better color match with the treated surface. Theaddition of Fe₃O₄, or AC, or CB will impart to the composition anydesirable shade of gray. If the stain occurs on a red brick surface,Fe₂O₃ can be added to the composition. The treatment of stains on beigeor light-brown surfaces may require the addition to the formulation of asmall amount of NiFe₂O₄.

The role of a binder (which is used in the second embodiment of theinvention) is to bind all the ingredients into a solid composition thatcan be formed into any desired shape, e.g., a rectangular bar, acylinder, a sphere, a cone, etc. Non-limiting illustrative examples of abinder are white Portland cement, gypsum, ceramic cement, stucco,marcite, etc.

Tables 1 and 2 below provide a generic liquid and solid phasecompositions of the present invention, respectively. TABLE 1 LiquidStain Masking and Removal Composition Component Function Concentrationrange, wt. % Titanium dioxide photocatalyst 0.1-40  tris(2,2′-bipyridyl) sensitizer 0.001-5    ruthenium (II) chloride Platinumdopant 0.001-5    Silica gel mediator  1-20 Oxalic acid co-reagent  1-20Iron oxide (Fe₂O₃) pigment 0.1-10  Ethanol solvent   40-99.9

TABLE 2 Solid Stain Masking and Removal Composition Ingredient FunctionConcentration Range, wt. % Titanium dioxide photocatalyst  1-70 tris(2,2′-bipyridyl) sensitizer 0.1-5   ruthenium (II) chloride, Tin oxide(SnO₂) dopant 0.01-5   Silica gel mediator  1-20 Oxalic acid co-reagent 1-40 Iron oxide (Fe₂O₃) pigment 0.1-10  White Portland cement binder30-99

In many cases, the same component of the composition can perform two oreven more functions. For example, TiO₂ can act as a photocatalyst and apigment; activated carbon (AC) functions as a mediator and a pigment;ferric oxide (Fe₂O₃) can be used as a dopant and a pigment.Advantageously, some components of the disclosed composition havepronounced anti-fungal characteristics, which may be beneficial intreatment of mold-related stains. For example, the anti-bacterial andanti-fungal effect of TiO₂, which destroys a cell membrane, are wellknown as disclosed by D. Blake et al, in Separation and PurificationMethods, vol. 28, pp. 1-50, (1999). Zinc oxide (ZnO) is widely used as amold-growth inhibitor in paints. Copper and silver, which could also actas dopants, are known for their anti-fungal and antibacterialproperties.

It should be noted that in some cases, the treatment of a stain by thedisclosed method is limited to a masking effect only, without completedecomposition and removal of the stain. This takes place when a stainingagent and the material of the stained surface are of similar chemicalnature, which results in fusing or absorbing the stain into thematerial. Widely occurring lube-oil stains on asphalt surface is a casein point. Both lube-oil and asphalt are predominantly high-molecularweight hydrocarbons, therefore asphalt readily absorbs lube-oil byvolume (in contrast, concrete adsorbs lube-oil by surface). This makesdecomposition and removal of lube-oil from asphalt practicallyimpossible. However, due to its good masking characteristics (highopacity, strong adhesion to the stain and durability), the coatingaccording to the present invention makes the lube-oil stain practicallyinvisible against the background for several months; after that it mayrequire additional coatings.

We note here that the inventive composition for masking and removingstains profoundly differs from conventional paints containing titaniumdioxide or other pigments. As discussed in the background section paintscan provide only masking effect, but they do not remove the stains.Paints contain a vehicle (such as oil- or lacquer- or latex-basedvehicles), in which the pigment is suspended. The vehicle dries andhardens to form an adhesive film of paint on the surface. This film ispractically permanent and over extended period of time (months andyears) its masking effect might diminish as the surroundings changecolor due to aging and exposure to traffic. In contrast, the novelcomposition does not contain any vehicle, and the coating willeventually weather out concurrent with the disappearance of the stain.Any residual coating may be easily washed away with brush and soap.

Reference is now made to FIG. 1, which illustrates the inventive conceptby providing a schematic diagram of the film composition applied to anoil stain occurring on a solid surface or substrate, 1. For the sake ofsimplicity, the shown composition comprises a plurality of photocatalyst3, a dopant 4 and a pigment 5 particles. As illustrated in FIG. 1, uponexposure to solar photons 6, the composition film degrades the stain 2into CO₂ and H₂O. The method is particularly advantageous for themasking and/or removal of the stains that are most difficult, sometimes,impossible to remove by conventional techniques and existing commercialproducts. The difficult stains include, but are not limited to, oldlube-oil stains and tire marks on rough porous surfaces such asconcrete, asphalt driveways, pavements and parking lots.

The disclosed method includes the application of a durable thin film ofa composition that is stable under exposure to elements, such as sun,wind, rain and mechanical action including, walking or driving over thefilm for extended period of time, such as, weeks and even months, duringwhich the stains usually disappear from sight. Referring now to FIGS.2A, 2B and 2C, there is an illustration of the three phases of the soilmasking and stain removal process of the present invention. In FIG. 2Athere is an original oil stain 20 on a concrete surface, FIG. 2B showsthe stain 22 coated with a thin film of the composition of the presentinvention and FIG. 2C shows the area of the stain 25 after exposure tothe elements for approximately 30 days. The stain is the same color asthe background surface.

With regard to the treatment of various rugged, solid surfaces, a ruggedsurface is not necessarily porous. For example, asphalt surface isrugged but not porous, unlike concrete and brick that have poroussurfaces. The minimum pore size is determined by the size ofphotocatalyst particles, such as approximately 30 nm to approximately1000 nm. There is no maximum measure of pore size, in fact, the surfaceswith larger pores are easier to treat because of better exposure tosolar photons.

The invention can also be used for a variety of indoor applications,including, but not limited to, a clean-up job in a garage, kitchen,patio, or swimming pool. For the indoor applications, there may be aneed for an artificial light source, such as, fluorescent white cool orblack light lamps. The light sources used for the removal of indoorstains, such as, in areas where exposure to direct solar radiation isnot possible, would require wavelengths in a range from approximately250 nanometers (nm) to approximately 800 nm, preferable, approximately365 nm to approximately 580 nm. The exposure time depends on thetoughness of the stain and may range from approximately 24 hours toapproximately 30 days.

The present invention offers many advantages over the prior art,including, but not limited to, a pronounced masking effect of thedisclosed composition film that causes a stain to become practicallyinvisible against the background immediately upon the application of thefilm. The method of using the composition is passive and care-free; noaction is necessary after applying the film, the stain willspontaneously disappear upon exposure to an outdoor environment withsunlight, air, and moisture. The application of the film is very simpleand hassle-free. The method of using the composition is applicable to agreat variety of organic and inorganic stains occurring on rough andrugged surfaces, both horizontal and vertical. There is no need forimmediate attention to the treated surface, such as, washing orscrubbing the surface, removing excessive chemical reagents, and thelike. In addition, all the reagents of the disclosed composition areinexpensive, widely available and safe to humans, animals and theecosystem.

All the above features make it very easy and convenient to treat avariety of stains on rugged surfaces by using a novel stain removal kit.In FIG. 3, the stain masking and removing kit is shown. The kit caninclude a dropper 31 (for the treatment of small areas), a sprayer 32and a pressurized spray can 33 (for the treatment of medium and largeareas), all filled with a liquid composition. The kit also includes aplurality of solid compositions in the form of cylindrical bars ofdifferent color 35 (looking like colored chalks) that are designed forthe treatment of stains occurring on colored surfaces (e.g., ornamentalconcrete, bricks and the like). Several pieces of cloths, sponges andother liquid absorbing materials 36, along with the bottle of a solvent37 are also included in the kit. The kit can contain both liquid andsolid formulations for the treatment of stains of different nature,e.g., lub-oil, tire mark, rust, mold and the like. All the abovearticles can be enclosed in a case 30 such as but not limited to a handcarriable box, a carton, a bag, and the like.

The examples below are non-limiting and for illustration purposes only.Other uses of the invention will be apparent from the disclosure to oneskilled in the art.

EXAMPLE 1

In this example, we used TiO₂ (manufactured by Degussa Corp.) with thefollowing characteristics: crystallographic modification: a mixture ofanatase and rutile (ratio about 3:1), surface area 50 m²/g; the averageparticle size 30 nm. 1 g of TiO₂ was added to 50 ml of denatured ethanol(obtained from Fisher Scientific) and thoroughly mixed to form asuspension. The obtained suspension is very stable: the powder did notprecipitate for a period of several weeks and a certain portion of TiO₂particles remained in suspended state for several months and even years.Several drops of this suspension were dropped on the surface of an oilstain on asphalt such that a thin film of TiO₂ covered the surface ofthe stain after evaporation of the solvent. The procedure may berepeated if after complete drying of the solvent, the oil stain stilllooks too dark against the surrounding area. FIG. 4A depicts the photoof an oil stain 40 on asphalt with a portion of the surface immediatelyafter the application of the TiO₂ film 42. To illustrate the contrastbetween treated and untreated surfaces, only a portion of the oil stainwas coated with the TiO₂ film 42. FIG. 4B shows the same stain 45 after5 months of the exposure to weather and heavy traffic. It is evidentthat the application of TiO₂ film has a pronounced masking effect on thelube-oil stain on asphalt, and this effect lasted for 5 months after asingle application of the film.

EXAMPLE 2

The TiO₂ suspension in ethanol, as used in Example 1, was dropped, usinga conventional dropper, on the surface of a lube-oil stain occurring ona concrete driveway. The suspension was dropped in the middle of thestain and moved outward, to prevent spreading the film beyond the edgesof the stain. One drop of the suspension covered about 2 cm². FIG. 5Ashows the photos of the original stain 50 and FIG. 5B show the treatedstain 55 after a month of exposure to the elements. Caution should beexercised when applying the coating and an effort should be made to notapply the coating beyond the edges of the stain. However, if the appliedcoating extends to an unstained area, or the coating is too brightagainst the background color of the surface to be cleaned, the excesscoating could be removed by brushing with soap and water.

EXAMPLE 3

Example 3 is similar to the Example 2, except that 5 ml of aqueouscolloidal solution of platinum (0.025 wt. % Pt) was added to the TiO₂suspension in ethanol. In this example, Pt plays a role of a dopant thataccelerates photodegradation of an oil stain. The colloidal Pt solutionwas obtained by mixing 2.5 ml of the aqueous solution of H₂PtCl₆ (0.1wt. %) with 2.5 ml of the aqueous solution (0.01 wt. %) of theprotective polymer (polyvinyl alcohol) followed by adding 0.1 g ofsodium borohydride (NaBH₄) to the mixture under well-stirred conditionsat room temperature until all hydrogen bubbles ceased to evolve. TheTiO₂—Pt suspension in ethanol was used for photodegradation of lube-oilstains on a concrete driveway. Pt nanoparticles impart a grayish colorto the TiO₂ suspension, which provides a better masking effect, if usedon a gray concrete surface.

EXAMPLE 4

In Example 4, the TiO₂ suspension in ethanol is similar to that preparedin Example 1, except that 0.2 g of zinc acetate (Zn(OAc)₂) was added tothe suspension. In this example Zn(OAc)₂ plays a role of a dopant thataccelerates photodegradation of an oil stain. The resulting suspensionin ethanol was applied to a lube-oil stain on a concrete driveway toform a film that efficiently degraded the stain over 3-4 weeks.

EXAMPLE 5

In this experiment, we used TiO₂ with the following characteristics:crystallographic modification, primarily anatase; surface area 25 m²/g;the particle size varied in the range of 0.1-11 μm. 1 g of TiO₂ wasadded to 50 ml of ethanol (denatured) to form a suspension. In contrastto Example 1, the suspension produced from this form of TiO₂ was notstable, and most of the particles precipitated in a matter of an hour. Apiece of cloth was partially soaked with this suspension, while shakingthe bottle with the suspension, and TiO₂ was rubbed on the surface of alube-oil stain on concrete. The treated stain was left exposed to theelements. In a matter of 4 weeks, the stain became practicallyindistinguishable against the background.

EXAMPLE 6

In Example 6, a white Portland cement (WPC) is used as a binder with thefollowing chemical composition (wt. %): CaO (67.1), SiO₂ (21.7), Al₂O₃(4.4), Fe₂O₃ (0.2), MgO (0.5), SO₃ (3.3), others (2.8). 6 g of WPC wasmixed with 4 g of TiO₂, the same modification of TiO₂ as used in theExample 3.5 g of water was added to the mixture to form a paste with theconsistency similar to that of yogurt. The paste was introduced into acylindrical plastic mold having a diameter of 1.2 cm and a length of 6cm and left undisturbed at room temperature for 24 hours. The solidWPC-TiO₂ composition having a cylindrical shape was released from themold and used for the treatment of a lube-oil stain on asphalt. Thelube-oil stain was shaded by the WPC-TiO₂ bar to cover whole surface ofthe stain, similar to drawing on the asphalt with a piece of a whitechalk. A piece of durable cloth partially soaked with rubbing alcohol(approximately 70% isopropanol, and approximately 30% water) was used torub the white powder on the surface of the stain until it becomes almostindistinguishable against the surroundings. FIGS. 6 A and 6B provide anillustration of Example 6, depicting the photos of an original stain 60and treated oil stain 65 on asphalt. For the sake of comparison, only aportion of the stain was coated with the composition film.

EXAMPLE 7

The same technique and the solid WPC-TiO₂ composition in Example 6 wasapplied to the treatment of a tire mark on asphalt. FIG. 7 is a photo ofthe tire mark 70 with a treated area 75 immediately after treatment.Where treated, the mark is almost indistinguishable from the unmarkedasphalt.

EXAMPLE 8

To the stain removal composition in Example 6, 0.2 grams (g) of Fe₃O₄(as a black pigment) was added to the dry WPC-TiO₂ mixture. Theresulting bar has a gray color, which provides better masking effect ongray concrete and asphalt surfaces, compared to the originalwhite-colored WPC-TiO₂ composition.

EXAMPLE 9

In Example 9, 0.2 g of acetylene black from Aldrich Chemicals (as ablack pigment) was added to the dry WPC-TiO₂ mixture of Example 6. Theresulting bar has a light-gray color, which provides good masking effecton concrete and asphalt surfaces.

EXAMPLE 10

In Example 10, 0.2 g of Fe₂O₃ (as a red pigment) was added to the dryWPC-TiO₂ mixture. The resulting bar had a red-brownish color, whichprovided good masking effect on red brick surfaces.

EXAMPLE 11

The stain masking and removal composition is similar to Example 6,except that 0.1 g of SnO₂ (as a dopant) was added to a dry WPC-TiO₂mixture. The resulting solid WPC-TiO₂—SnO₂ composition more efficientlydegraded a lube-oil stain compared to the original composition (i.e.,without a dopant).

EXAMPLE 12

The stain masking and removal technique and composition similar to thatin Example 6 is used, except that 4 g of ZnO (surface area 13.4 m²/g;the average particle size 0.5-1 μm) was mixed with 6 g of WPC to form adry WPC-ZnO mixture. The resulting WPC-ZnO composition efficientlyphoto-degraded lube-oil stains on concrete surface in a manner that iscomparable to WPC-TiO₂ compositions.

EXAMPLE 13

Conditions were the similar to those in Example 6, except that 5 g ofTiO₂ (the same reagent as in Example 5) was mixed with 5 g of gypsum(Plaster of Paris) to form a dry gypsum-TiO₂ mixture. Water was added tothis mixture to form a paste with the consistency of yogurt. The pastemixture is allowed to harden in a mold. A lube-oil stain was shaded bythe solid composition to cover the whole surface of the stain, similarto drawing on the asphalt with a piece of a white chalk. A piece ofdurable cloth partially soaked with rubbing alcohol (approximately 70%isopropanol, and approximately 30% water) was used to rub the powder onthe surface of the stain until it becomes almost indistinguishableagainst the surroundings. The resulting gypsum-TiO₂ solid compositionefficiently photo-degraded lube-oil stains on concrete and stonesurfaces.

EXAMPLE 14

Example 14 relates to the treatment of a rust stain on concrete surface.Two (2) g of oxalic acid was mixed with 10 g of gypsum and 1 g of TiO₂,the same reagent as in Example 5. Water was added to this mixture toform a paste with the consistency of yogurt. The paste mixture isallowed to harden in a mold. A rust stain was shaded by the solidcomposition to cover the whole surface of the stain, similar to drawingon the concrete with a piece of a white chalk. A piece of durable clothpartially soaked with rubbing alcohol (approximately 70% isopropanol,and approximately 30% water) was used to rub the powder on the surfaceof the rust stain until it becomes almost indistinguishable against thesurroundings. The resulting solid composition efficiently removed therust stain from the concrete surface.

The experiments above have demonstrated that the kit form of theinvention shown in FIG. 3 can be used to apply the novel composition tomost stains in less than approximately 30 minutes, and preferably inless than approximately 5 minutes. In less than approximately 180 daysand preferably in less than approximately 30 days, the entire stain canbecome distinguishable against the surroundings.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1. A method for masking and subsequent removal of unsightly stains fromrugged solid surfaces comprising the steps of: (a) providing a rugged,solid surface having a consistent color with a stain of a differentcolor than the surface; (b) coating the entire stain with a durable thinfilm of a composition that masks the stain; and (c) exposing the coatedstain to an environment that degrades and removes the stain from thesurface.
 2. The method of claim 1, wherein the composition includes oneor more components selected from a photocatalyst, a sensitizer, adopant, a mediator, a co-reagent, a pigment and a binder.
 3. The methodof claim 2, wherein the composition includes a photocatalyst combinedwith at least one different component selected from a sensitizer, adopant, a mediator, a co-reagent, a pigment and a binder.
 4. The methodof claim 2, wherein the photocatalyst is selected from the group ofcompounds containing zinc (Zn), titanium (Ti), molybdenum (Mo) andtungsten (W).
 5. The method of claim 4, wherein the photocatalyst isselected from one of titanium dioxide (TiO₂), zinc oxide (ZnO), and amixture of TiO₂ and ZnO.
 6. The method of claim 2, wherein the dopant isselected from the group of metals and compounds consisting of platinum(Pt), palladium (Pd), ruthenium (Ru), iron (Fe), copper (Cu), cobalt(Co), zinc (Zn), tin (Sn), and silver (Ag).
 7. The method of claim 2,wherein the mediator is selected from the group consisting of zeolites,activated carbon, carbon black, acetylene black and silica gel.
 8. Themethod of claim 2, wherein the pigment is selected from the groupconsisting of TiO₂, ZnO, Fe₂O₃, Fe₃O₄, NiFe₂O₄, activated carbon andcarbon black.
 9. The method of claim 2, wherein the co-reagent isselected from the group consisting of oxalic acid, citric acid, sodiumcitrate, potassium hydrogen tartrate, sodium phosphate, sodiumhydrosulfite and silicic acid.
 10. The method of claim 2, wherein thebinder is selected from the group consisting of white Portland cement,Portland cement, gypsum, ceramic cement, stucco and marcite.
 11. Themethod of claim 10, wherein the binder is white Portland cement.
 12. Themethod of claim 1, wherein the durable thin film is produced by applyinga suspension of the composition in a solvent.
 13. The method of claim12, wherein the solvent is selected from the group consisting of water,ethanol, acetone, isopropanol, octane and a mixture thereof.
 14. Themethod of claim 12, wherein the suspension of the composition is appliedby spray-coating onto the stain surface.
 15. The method of claim 12,wherein the suspension of the composition is applied by soaking aliquid-absorbing material with the suspension and rubbing thesuspension-soaked material on the stain surface.
 16. The method of claim1, wherein the coating step includes the step of: applying thecomposition in the form of a loose powder or pellets followed by thestep of rubbing the composition on the stain surface with asolvent-soaked material.
 17. The method of claim 1, wherein the coatingstep includes the step of: shading the stain with the composition in asolid form followed by the step of rubbing the composition on the stainsurface with a solvent-soaked material.
 18. The method of claim 1,wherein the environment that degrades and removes the stain is a lightsource and at least one of air, rain, humidity, pedestrian traffic andvehicular traffic.
 19. The method of claim 18, wherein the light sourceis solar radiation.
 20. The method of claim 18, wherein the light sourceis selected from one of a black light and white cool fluorescent lamps.21. The method of claim 1, wherein the stain of a different color thanthe surface is masked in a period of less than approximately 30 minutes,and the stain is removed in a period of time less than approximately 180days.
 22. The method of claim 21, wherein the stain of a different colorthan the surface is masked in a period of time up to approximately 5minutes and removed in a period of time of less than approximately 30days.
 23. A stain masking and removing kit comprising: a separatephotocatalyst; a separate liquid solvent; a separate pigment wherein auser unites a portion of the photocatalyst with a portion of the liquidsolvent and a portion of the pigment to match the color of a backgroundsurface that has a stain of a different color than the backgroundsurface; and an applicator for applying the photocatalyst, the liquidsolvent and the pigment to a surface.
 24. The kit of claim 23, whereinthe photocatalyst is combined with at least one of a sensitizer, adopant, a mediator, a co-reagent, a pigment and a binder.
 25. The kit ofclaim 23, wherein the liquid solvent is selected from the groupconsisting of water, ethanol, acetone, isopropanol, octane and a mixturethereof, and used to create a uniform coating of the photocatalyst onthe surface of a stain.
 26. The kit of claim 23, wherein pigment isselected from the group consisting of TiO₂, ZnO, Fe₂O₃, Fe₃O₄, NiFe₂O₄,activated carbon and carbon black.
 27. The kit of claim 23, wherein theapplicator is selected from the group consisting of a brush, sponge,cloth, sprinkler, sprayer, pressurized spray can and dropper.
 28. Thekit of claim 23, wherein a stain of a different color from thebackground surface is masked in a period of less than approximately 30minutes, and becomes removed in a period of time of less thanapproximately 180 days.
 29. The kit of claim 28, wherein the stain ismasked in a period of less than approximately 5 minutes and becomesremoved in less than approximately 30 days.
 30. A stain masking andremoving composition, comprising in combination: a photocatalyst havinga concentration range of approximately 0.1 to approximately 70 weightpercent; a sensitizer having a concentration range of approximately0.001 to approximately 5 weight percent; a dopant having a concentrationrange of approximately 0.001 to approximately 5 weight percent; amediator having a concentration range of approximately 1 toapproximately 20 weight percent; a co-reagent having a concentrationrange of approximately 1 to approximately 40 weight percent; a pigmenthaving a concentration range of approximately 0.1 to approximately 10weight percent; and a binder or a solvent having a concentration rangeof approximately 30 to approximately 99.9 weight percent, wherein thecomposition is used for masking and removing a stain from a surroundingsurface.